JPH0359949B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field This invention relates to scintillator materials. The invention is particularly useful with regard to the production of ultrafast detectors of high energy photons, ie X-rays or gamma photons, and the production of tomographs.

(Background technology) One of the most accurate means of measuring photon travel time
One type uses a high-speed photomultiplier tube and a plastic scintillator. There are many commonly used plastic scintillators, such as PILOT U or NE111. During the unexcited state due to interaction with gamma photons, all plastic scintillators emit light pulses with a time constant of 1.5 ns. The strength of this pulse, ie the total number of photons in the pulse, is proportional to the energy of the incident gamma photons. A light output of approximately 3000-4000 photons/MeV can typically be obtained via the plastic scintillator.

The apparatus shown in FIG. 1 is used to determine the temporal change or difference in time of flight of a plastic scintillator and to compare the plastic scintillator with other scintillators.

The device has opposed detectors D ₁ , D ₂ consisting of thin plastic scintillators 2a, 2b and fast photomultipliers 3a, 3b. High-speed photomultiplier tubes 3a and 3b are electrically connected to high voltage power supplies 4a and 4b, respectively. Furthermore, they are electrically connected to electrical amplifiers, pulse height analyzers and counting means, which are well known to those skilled in the art. This device is capable of measuring the time difference between the detection time of a gamma photon detected from the first detector D ₁ and the detection time of another gamma photon detected from the second detector D ₂ . These two γ photons are each
It has an energy of 511 keV, is generated by the annihilation of an electron and a positron, and is emitted in opposite directions from the point 6 of the object 1 having the positron-emitting substrate.

The measurement results of these time differences are variable and exhibit a Gaussian distribution that varies depending on the type of scintillator used in the measuring device in which the photomultiplier tubes 3a, 3b and the electrical means 5 are cascaded. This distribution indicates the full width at half maximum (FWHM) of the spectrum.

For a plastic scintillator with a diameter of 20 mm and a thickness of 2 mm, the FWHM is approximately 150 ps (the energy threshold is chosen such that the event occurs only due to the photoelectric effect) and the
By utilizing 90%, it becomes almost 200 ps (the energy threshold is more than 100 keV).

The main drawbacks of plastic scintillators are that the plastic material is made up of elements with low atomic numbers and that the plastic material has a low density (approximately 1.1-1.2 g/cm ³ ).

Therefore, the phenomenon governing the interaction of gamma photons with the plastic material in this case is the Compton effect. That is, there is no relationship between the energy of incident gamma photons and the energy generated in the plastic scintillator. As a result, spectroscopic measurements of gamma radiation are not possible. Therefore, it is possible to annihilate a large number of interactions without losing a small number of γ photons that exhibit interactions due to the photoelectric effect, or to analyze all photons that have undergone interactions in addition to those that can be analyzed as the energy of γ photons. There is a need to.

Another prior art technique for measuring photon transit times with great precision is a scintillator using cesium fluoride, CsF. This technique is described in particular detail in French patent application No. 79 02053. Cesium fluoride CsF has an atomic number of 55
It has a large cesium atom and a high density (almost
4.6g/cm ³ ). However, cesium fluoride is inferior to plastic in the following two respects. That is, the fluorescence time constant is approximately 2.5 to 3 ns.
(approximately 1.5 ns for plastics), and this scintillation response is approximately 1500-2000 photons/MeV (approximately 3000-4000 photons/MeV for plastics).
However, cesium fluoride scintillates at an interesting energy level.
When using 511keV gamma photons, the resolution is approximately 30
%. Furthermore, if the plastic scintillators 2a and 2b of the apparatus shown in FIG.
FWHM is obtained.

Therefore, the time performance of plastic scintillation is better than that of cesium fluoride. however,
Considering the fact that the stopping power of cesium fluoride is higher than that of plastic, cesium fluoride shows better time performance than plastic when obtaining the same detection effect.

However, when compared to plastic,
Very high hygroscopicity of cesium fluoride – e.g. thallium-doped sodium iodide NaI (Tl)
Cesium fluoride is extremely difficult to handle because of its hygroscopicity, which is higher than that of cesium fluoride. Therefore, when cesium fluoride is used in the production of scintillators, this substance must be stored in a very strong container. Therefore, cesium fluoride is very expensive and difficult to handle.

(Purpose of the Invention) The purpose of the present invention is to solve the problems of the prior art, and its feature is to use a scintillator material made of barium fluoride. It is a pure scintillator material that emits light with a slow component of 3200 Å and a fast component of the maximum center of 2250 Å, and in order to reduce defects in the crystal lattice and increase the strength of the fast component.

For example, the invention is useful in the production of ultrafast high energy photon (X or gamma photon) detectors, particularly those seeking time-of-flight measurements. This invention is also useful for producing positron tomography and time-of-flight tomography.

(Structure and operation of the invention) This invention will be explained in detail below.

Barium fluoride (BaF ₂ ) scintillator is
It exhibits a level of temporal variation that is nearly identical to that of plastic scintillators, has notable advantages over cesium fluoride scintillators, is easier to use, and is less expensive than plastic scintillators. One method for purifying barium fluoride that can be used to obtain extremely pure metal halide single crystals is disclosed in the US Patent No.
Described in No. 4110080. In practice, the substances used in the invention are, for example, Laboratories Merck-Clvenot,
65-67, Rue De La Victory
Commercially available from Victor Victore, 75009 Paris, France. Barium's atomic number, 56, is slightly larger than that of cesium, 55. Therefore, the density of cesium fluoride is 4.61
g/ ^cm3 , whereas the density of barium fluoride is
It is 4.88g/ ^cm3 . The stopping power of barium fluoride with respect to X-rays or gamma photons is higher than that of cesium fluoride. Barium fluoride is also resistant to water and many oxidizing solvents such as ethanol, ethyl ether, acetone, and methanol. Furthermore,
Barium fluoride is easy to process, and although it is harder than glass, it is not brittle. Therefore, the first
Plastic scintillator 2 of the device shown in the figure
By replacing a and 2b with barium fluoride having the same shape, it is possible to obtain a FWHM of approximately 200 ps regarding the time performance of barium fluoride.

These values can be explained by the scintillation properties of barium fluoride. The radiation of this material consists of a fast component - time constant: approximately 0.8 ns, optical output: approximately 1000 photons/MeV - and a slow component - time constant: approximately 600 ms, optical output: approximately 3000-4000 photons/MeV.
It consists of MeV. The high speed component is used in the travel time measuring device to obtain good time performance. The energy resolution of barium fluoride can be utilized similarly to cesium fluoride with the same or better results. Energy analysis is performed in synchronization with time analysis, and the high-speed component of 1000 photons/
Just consider MeV. For example, when using 511 keV photons, the energy resolution of the high-speed component is approximately 30%. This energy resolution is the same as that of cesium fluoride. It is also possible to delay the energy analysis until the light emission is completely finished, in particular for 1.5-2 μs. This can be confirmed from the analysis results. On the other hand, the analysis time result is
It is held for a few μs in a suitable analog or digital device. In this case, the energy resolution for 511 keV photons is approximately 13%. This is very advantageous for positron tomography and the like. However, the presence of scattered radiation is extremely disadvantageous.

The existence of the slow and fast components, the existence of a time constant, and the measurement of the emission spectrum of the barium fluoride scintillator reveal the following: The slow component is
Defects in the crystal lattice of BaF ₂ are caused by chemical impurities or insufficient alignment of barium and fluorine in the lattice. These defects lead the intermediate level into the forbidden band and give rise to radiation with a higher wave height than that due to the fast component with a high time constant (0.6 μs). Experiments have shown that this fast component corresponds to a radiation band between 2000 and 2500 Å, with a maximum center at 2250 Å. The slow component has a peak value of approximately 2500 Å, and this peak value can reach 6000 Å due to the action of the BaF ₂ sample. The maximum intensity of the optical radiation is approximately 3200 Å.

In the wave height region of the maximum center of 2250 Å, the transmittance of the BaF ₂ sample is closely related to the radiation of high-speed components. Therefore, a sample that exhibits absorption at 2250 Å in its travel spectrum has no or almost no fast component in its emission spectrum. This phenomenon is directly related to the height of the absorption peak of the sample. As a result, the larger the absorption peak (for a given sample thickness), the fewer photons are emitted at high speed.

Therefore, for a given property (ie, for a given impurity level), the thicker the thickness, the fewer photons will be emitted at high speed.

Thus, the "inherent" emission of BaF ₂ at 2250 Å is found in all samples made from this material. However, this radiation can be masked by absorption by defects in the BaF ₂ lattice.
This defect retards the light emission, changing it to radiation with a higher wave height.

Therefore, by improving the purity of barium fluoride (e.g. by removing chemical impurities, especially ions, and removing lattice defects), the intensity of the fast component relative to the slow component can be reduced when barium fluoride emits light.
It can be made even stronger.

Pure barium fluoride can be used to make scintillators for tomography, especially positron tomography and time-of-flight detectors. The positron tomography is described in detail in French Patent Application No. 7902053. This is called “backward radiation”
The reduction in length or "backward radiation" distance benefits positron tomography when compared to transit time tomography and conventional positron tomography. Therefore, the noise present in the reconstructed image for the same number of detection results is reduced. This advantage is clear from the comparison of the number of detection results, with respect to time of flight, positron tomography and conventional positron tomography.
The same characteristics can be obtained.

The curves in Figure 2 were determined from a positron source time-of-flight measurement device for different diameters φ of phantoms uniformly distributed on a positron-emitting substrate.
FWHM (expressed in ms) and time-of-flight tomograph,
The relationship with the gain change G (represented by the counting rate of the detection result) obtained from the positron tomography is shown. These curves demonstrate the importance of FWHM improvement. Therefore, as the FWHM decreases, the gain increases, and the amount of tracer injected into the substance to be measured in a time-of-flight tomography or positron tomography becomes smaller than in the case of a normal positron tomography.

By using pure barium fluoride scintillator obtained by the method described above,
Compared to the time-of-flight measurement obtained by using a cesium fluoride scintillator as a scintillator of the same shape, at least the measurement error is reduced.
It is possible to obtain a FWHM that is about 1.3 times smaller.

Apart from this time measurement advantage, barium fluoride has a somewhat larger attenuation coefficient than cesium fluoride. This is because the atomic number of barium is larger than that of cesium, and the density of _BaF2 is
This is because it is higher than CsF. In addition, by using barium fluoride, time-of-flight tomography,
The sensitivity of positron tomography can be improved by approximately 5-10%. Furthermore, as mentioned above, barium fluoride has no affinity for water. Unlike cesium fluoride, it does not need to be sealed in a container (therefore expensive and difficult to use) for scintillator production. A novel scintillator material using barium fluoride can reduce surface losses between scintillators and improve sensitivity by approximately 15% for time-of-flight and positron tomography.

Therefore, pure barium fluoride is superior to cesium fluoride in terms of temporal behavior, detection efficiency, ease of use, and scintillator manufacturing cost. For time-of-flight measurements, the use of barium fluoride in the manufacture of the detector of a positron tomography provides a coefficient of at least In order to perform the test with a factor equal to 1.5, the amount of tracer injected into the substance to be measured can be reduced.

[Brief explanation of drawings]

Figure 1 is a diagram of the configuration of the apparatus for determining the temporal change of the scintillator, and Figure 2 shows the results obtained from the positron point source time-of-flight measuring apparatus for different diameters φ of the phantom with a constant distribution of the positron-emitting substrate.
It is a graph showing the relationship between FWHM and gain change G obtained from a time-of-flight tomography and a positron tomography. 1... positron emission source, 2a, 2b... scintillator, 3a, 3b... photomultiplier tube, 4a, 4b
...High voltage power supply, 5...Electrical amplifier.

Claims (1)

[Claims] 1. An ultrafast scintillator that detects high-energy photons, which contains a barium fluoride material, and the purity of the barium fluoride material is transparent to light in a wavelength range from 2000 Å to 2500 Å. is excited by the high-energy photon and emits a first component and a second component of light, the first component being a slow component and having a wavelength of 2500 Å or more, and the second component having a wavelength of 2000 Å. An ultra-high-speed scintillator characterized by its high-speed component purity in the wavelength region from to 2500 Å. 2. Claim 1, wherein the scintillator is used for measuring the time of flight of photons.
The ultrafast scintillator described in section. 3. An ultrafast scintillator for detecting high-energy photons, which includes a barium fluoride material, the barium fluoride material has a purity that is transparent to light in a wavelength range of 2000 Å to 2500 Å, and the high-energy scintillator Excited by photons and emitting a first component and a second component of light, the first component being a slow component and having a wavelength of 2500 Å or more, and the second component having a wavelength range from 2000 Å to 2500 Å An ultra-high-speed scintillator characterized by its purity, which is a high-speed component of 4. The ultrahigh-speed scintillator according to claim 3, wherein the tomograph is a positron tomograph. 5. The ultrahigh-speed scintillator according to claim 4, wherein the tomograph is a time-of-flight tomograph.